In general terms, codebook based transmission in combination with multiple dual-polarized antennas generate radiations patterns that are the result of coherent free-space summation of the signals from co-polarized antenna elements and incoherent (power sum) summation of signals from orthogonally polarized antenna elements. Thus, generally, the relative phase values (and amplitude values) of signals intended for co-polarized elements will influence the shape of the resulting patterns.
A combined unit consisting of a separate phase-coherent four-channel radio connected to a separate phase-coherent array antenna, with all phase delays assumed known, generates radiation patterns that can be deterministically predicted provided that no other sources of phase delays are present. However, when the connections between radio and antenna, primarily cable connections, are installed on-site, no knowledge of the additional cable phase delay can be assumed to be available. In addition, the antenna may be an off-the-shelf product with unknown internal phase properties. Hence, there may be a risk that the resulting radiation patterns are unknown. Further, the resulting radiation patterns may potentially have undesirable radiation properties if the radio and antenna are combined without taking the phase delays into account. This issue may be of particular relevance for single-stream transmission of cell-covering (legacy) signals.
A four-channel radio for multi-layer transmission should also support single-stream transmission of legacy signals with uniform load-balancing over the power amplifiers. With unknown phase delays (cable lengths) between the radio and a multi-column array antenna, the single-stream signal may experience unknown phase rotations for each antenna port, resulting in array antenna patterns with unknown shape and pointing direction.
Assuming unknown phase values for each cable connection, one way to guarantee desired antenna pattern characteristics involves calibration of the radio-cable-antenna signal paths such that any variations in phase between the different signal paths can be compensated for. However, calibration of the entire set of radio, cable, and antenna transmission chains requires that the radio supports calibration and that the array antenna is equipped with calibration hardware (such as branchline couplers and calibration port). The former adds complexity and the latter is typically not the case for off-the-shelf antennas.
Another way to guarantee desired antenna pattern characteristics involves usage of implicit indicators of coverage performance, for example based on feedback signaling from wireless terminals to the network node, to control the relative phase settings of the different signal paths at the network node. Such implicit indicators of coverage performance may adjust the system to perform well in terms of the feedback measures, but only for the set of locations occupied by active wireless terminals. This means that the actual coverage area is unknown, which can result in coverage holes, for example along sector borders between adjacent co-sited antennas.
The international patent application WO 2011/005162 A1 discloses a system with four (parallel) power amplifiers feeding a dual-column, dual-polarized array antenna where a signal from each of four effective ports will be amplified by all amplifiers. The international patent application WO 2012/166030 A1 relates to creating (near) orthogonal power balanced ports for transmission using 1, 2 and 4 transmitters. A phased matched system is used to guarantee well shaped sector covering beams. However, there is still a need for an improved load balancing of dual-polarized antennas.